The deterioration of a well begins as soon as the well is put into service. A sound preventive maintenance program can go a long way in extending the life of a well and lessening repair costs in the future. A well may fail due to incrustation, chemical corrosion, or mechanical corrosion. Field experience has shown that these types of failures are usually interrelated. (Jay Lehr, et al., Design and Construction of Water Wells, 162) Incrustation in a well is defined as the deposition of organic or inorganic material within the aquifer near the borehole; in the gravel pack; around the screen; upon surfaces of the screen; casing, or pump; or any combination of these. The resulting deposits restrict the water passages from the aquifer into the well and increase surface roughness. This in turn increases local ground water velocities, causing a strong flow and subsequently reducing the specific capacity of the well. Listed below are the three types of incrustation defined. 1. Chemical Incrustation: This type of incrustation is dependent on the ground water quality and the design of the well. Soluble materials such as iron, manganese hydroxides and hydrated oxides found in ground water will precipitate out of solution, causing chemical incrustation. When water is pumped from the aquifer the water table declines, lowering the hydrostatic pressure exerted on the ground water. This reaction upsets the chemical equilibrium of ground water in that immediate area. Because of the hydrostatic pressure drop, carbon dioxide will be released and the soluble bicarbonate carried in the ground water will convert to the insoluble carbonate form. The insoluble carbonate is deposited on the face of the screen, plugging the holes and reducing the yield of the well. 2. Physical Incrustation: This type of incrustation involves the clogging of screen openings with solid particles carried from the aquifer. This condition results primarily from the failure to effectively remove fine particles from the gravel pack and surrounding aquifer during construction of the well. Physical clogging is a result of improper well design. Slot geometry of the screen may encourage sand grains to be trapped in the slot opening, thereby decreasing the effective opening area. A screen that is slightly smaller on the outside than on the inside helps to eliminate this problem. 3. Biological Incrustation: A variety of organisms are capable of deriving their nutrition from ground water containing dissolved iron. Some organisms can convert iron bicarbonate to ferric ions and consume the resulting carbon dioxide. Others use organic compounds found in ground water as food and release ferric ions as a byproduct. Other biological organisms appear to convert ferrous ions to the ferric ion form when either organic or inorganic compounds of iron are present in ground water. The result of these events is the development of a complex deposit consisting of a reddish brown hydrated iron oxide, a black oxide film, and a gelatinous mass of living organic matter. Both the precipitation of iron and the rapid growth of bacteria create a voluminous mass of material that quickly plugs the water passages of the adjacent aquifer, gravel pack, and well screen. Iron bacteria favor environments within the well system that create a high flow velocity environment. The pump intake, column pipe, and well screen are prone to development of iron bacteria colonies. There are several theories regarding the development of iron bacteria. One is the bacteria is already present in the aquifer and the change of the environment due to a pumping water well causes them to spread. Another theory is that iron bacteria spores are transported from well to well through drilling and pump handling equipment. Once a colony is established, bacteria spores can be transferred between wells. (Jay Lehr, et al., 163-165) Prevention of Incrustation There are several modifications to the water well system that may reduce the potential for biological and mineral incrustation: A. Increase the open area of the well screen: By increasing the open area of the screen, ground water enters the well more freely and friction, head loss, and velocity of the ground water are all reduced. These reductions reduce the tendency for chemical precipitation and scaling to occur. B. Improve the efficiency of the well: Ground water head loss through the well screen is reduced by improving the efficiency of the well. The potential for chemical precipitation development is reduced when a well is more efficient. C. Reduce the pumping rate of the well system: By spreading the pumping load over several wells, the pumping rate from an individual well is decreased and drawdown will be decreased. A decrease in the drawdown of a well reduces the head loss, friction loss, and velocity of the ground water as it enters the well. D. Follow a proper preventive maintenance program: By monitoring the physical and chemical changes of a well, trends in the performance of a well can be obtained. This data may allow the well operator to predict an incrustation problem before it occurs. A proper maintenance program can prevent an incrustation problem from becoming critical. E. Reduce the oxygen entering the well system: Bacterial growth and chemical precipitation may be dependent on the presence of oxygen. The oxygen level can be decreased by reducing the drawdown in a well, which increases the pumping water level, preventing air from being dissolved in the ground water near the screen. A drawdown seal can be installed in the well, creating a physical barrier between the atmosphere and the ground water in the well. A drawdown seal is a plate that seals itself against the casing. It is installed just above the pump and is designed to allow water from the pump to pass through it without allowing air to enter the well. Corrosion is the chemical or physical decomposition of materials and several types of corrosive forces can affect a well. Chemical and galvanic corrosion and physical erosion are the most common forces that are at work in a well. Chemical corrosion is the process that removes metal ions from the surface of the corroding metal. Electrochemical corrosion means that a chemical change is accompanied by the flow of electrical current. This type of corrosion process requires considerable salts in the ground water to make the metal an excellent electrolyte. There are several water quality issues that can indicate possible corrosive conditions in a well. They are; acidic water (ph less than 7), dissolved oxygen, hydrogen sulfide, total dissolved solids, carbon dioxide, chlorine, higher temperature increases corrosion by accelerating hydrogen evolution. By closely matching the material used in the well screen to each ground water environment, the life of a well can be greatly extended. (Jay Lehr et al., 166-168) Troubleshooting Well Problems The performance history of a well is the starting point for determining what the problem is or where it originated. It is important to keep accurate performance records of a well to aid in understanding and correcting well performance issues. There are three tools that can be used to troubleshoot well problems. Historical data and actually seeing the problem are two options. The third option is a water quality analysis of a water sample to be used for comparison purposes. Below is a list of problem solving methods based on field observations: Observed: Heavy reddish-brown iron oxide, stains in discharge, or red water. Possible Conditions: Indicates anaerobic conditions (a reducing environment). There may be a favorable environment for either inert or biological iron deposits and corrosive waters may be attacking the metal parts of the well. There may also be iron bacteria, which are creating the problem. Aerobic conditions, meaning an oxygen enriched environment may indicate the yield of the well has dropped, a possible hole in the casing, or iron oxide scaling. Course of Action: Test chemical and bacterial content of water and perform a video camera survey of the well to see bacterial growth, scale, or cascading water.
Observed: Bubbles in the discharge water. Possible Conditions: An environment with free carbon dioxide, cascading water, naturally dissolved gases in the water, and over pumping of the aquifer may be the cause of the problem. Course of Action: Test chemical and bacterial content of water and perform video camera survey of the well to see bacterial growth, scale, or cascading water.
Observed: Rotten egg smell (hydrogen sulfide odor): Possible Conditions: A reducing environment. Course of Action: Test chemical and bacterial content of water for hydrogen sulfide concentrations. If levels are 0.5 mg/l or greater, damage may result to copper alloy parts of the well system. A video camera survey of the well to see bacterial growth and effects of a corrosive environment can be performed.
Observed: Well efficiency decreased. Possible Conditions: Chemical and mechanical incrustation, biological fouling, decrease in regional water table, structural collapse caused by corrosion or other factors, change in water quality, improper well design and construction, and pumping in excess of design. Course of Action: Test chemical and bacterial content of water. A video camera survey of the well to see bacterial growth, scale, and cascading water can be performed. Test pumping system and well and measure water levels after well recovers and during pumping.
Observed: Reduced production and loss of pressure. Possible Conditions: Pumping system deteriorated, change in pumping water level, reduced static level, decrease in well efficiency. Course of Action: Test pumping system and well, measure water levels after well recovers and during pumping, measure pump shut-off head, measure wire-to-water efficiency of the pumping system, and inspect pumping system.
Observed: Sand in discharge, loss of pressure and land subsidence around well. Possible Conditions: Inadequate or improper well design and construction, well screen and casing deterioration caused by corrosion, and well being pumped in excess of design intent. Course of Action: Review well design, review well construction for conformity to design, and measure water levels after well recovers and during pumping.
Observed: Pump is providing variable discharge and broken suction. Possible Conditions: Well being pumped in excess of design intent, drawdown level excessive in well because regional water level has declined or well efficiency decreased, well screen and casing deterioration caused by corrosion, and well screen encrusted. Course of Action: Test pumping system and well, measure water levels after well recovers and during pumping, inspect pumping system, and test chemical and bacterial content of water.
Observed: Reduced yield of well. Possible Conditions: Plugging of well outside of borehole in the aquifer. Course of Action: Test chemical and bacterial content of water, perform video camera survey of well to see bacterial growth, scale and cascading water, test pumping system and well, measure water levels in remote wells, and measure static water levels for region.
Observed: Reduced yield of well. Possible Conditions: Regional drought, overuse of the resource, recharge area for aquifer is reduced or damaged. Course of Action: Test pumping system and well, measure water levels in remote wells, measure static water levels for region, test chemical and bacterial content of water, perform video camera survey of well to see bacterial growth, scale, and cascading water.
Observed: Reduced yield of well. Possible Conditions: Recharge areas reduced because of seasonal changes, permanent effects to recharge areas. Course of Action: Test pumping system and well, measure water levels in remote wells, measure static water levels for region, test chemical and bacterial content of water, perform video camera survey of well to see bacterial growth, scale, and cascading water, and review well design.
(Jay Lehr et al., 168-171)